1. Field Of the Invention
This invention relates generally to systems and techniques for performing wire and cable testing. More specifically, the invention teaches how to utilize the principles of spread spectrum signal transmission and reception to perform wire and cable testing in aging aircraft where wires and cable can be difficult to access, including determination of wire or cable characteristics such as length and impedance, which is characterized as an open or short circuit condition.
2. Description of Related Art
The benefits of being able to test wires and cables (hereinafter to be referred to as a wire or wires) are many. Some reasons are obvious. For example, wires are used in many pieces of equipment that can have catastrophic results if the equipment fails. A good example of this is an airliner. However, the consequences of non-performance do not have to be so dire in order to see that benefits are still to be gained. For example, wires are used in many locations where they are difficult to reach, such as in the infrastructure of buildings and homes. Essentially, in many cases it is simply not practical to remove wire for testing, especially when this action can cause more damage than it prevents.
Given that the need for wire testing is important and in some cases imperative, the question is how to perform accurate testing that is practical, meaning relatively inexpensive and at a practical cost. The prior art describes various techniques for performing wire testing. These techniques include but are not limited to time domain reflectometry (TDR), frequency domain reflectometry (FDR) and standing wave reflectometry (SWR). TDR is performed by sending an electrical pulse down a wire, and then receiving a reflected pulse. By analyzing the reflected pulse, it is possible to determine wire length, impedance, and the location of open or short circuits.
In standing wave reflectometry (SWR), a signal is transmitted and a reflected signal is received at a directional coupler. The system then measures the magnitude of the reflected signal. A short circuit, an open circuit, and the depth of a null give the same information as TDR. However, this technique is less generally accurate and nearly as expensive.
The process of frequency domain reflectometry (FDR) can be summarized as generating an input signal, splitting the input signal to the wire under test and to a mixer, also sending a reflected input signal to the mixer to thereby generate a mixed signal, removing or ignoring high frequency components, digitizing a remaining component that contains information regarding impedance and length of the wire under test, performing the same steps for several different frequencies, and analyzing the plurality of digitized signals to thereby determine impedance and length of the wire under test.
Aging aircraft wiring often experiences faults during flight that do not occur on the ground. This is because during flight conditions, the conditions of operation are changed. These conditions include different temperatures, levels of humidity, physical and electrical stresses on the wires, etc. These so-called intermittent faults are extremely difficult to manage because they may not be detectable under testing conditions, and thus they are not resolved.
Wires carrying digital data are particularly problematic as it is critical that the testing process not interfere with the data being carried by the wires. Unfortunately, the wire testing apparatus and methods of FDR, TDR and SWR can not be used to perform testing without causing at least some type of interference.
To understand the present invention, it is necessary to now discuss an unrelated concept used in communications. It is observed that Direct Sequence Spread Spectrum (DSSS) is a common method used to improve performance of wireless communication devices. In DSSS, a pseudo-random noise code (PN code) is multiplied by the original digital data signal that is to be transmitted. The result is a newer and higher bandwidth data signal. This higher bandwidth data signal is transmitted via wireless link using methods like BPSK, QPSK, etc. as is known to those skilled in the art. The advantages of this type of transmission include efficient use of bandwidth, noise immunity, resistance to jamming, etc.
What is desired is an improved method of wire fault detection. It would be an improvement over the state of the art if the detection method could be used on live wires, wherein the detection method would not interfere with normal operation of the live wires.